The research objective of this application is to develop a detailed molecular and mechanistic understanding of the heterogeneity in the rheological and adherence properties of circulating sickle red cells and define the in vivo pathologic sequelae of these cellular changes. To achieve our objective we propose the following series of studies: 1) Determine the structural basis for the heterogeneous rheological properties of distinct subpopulations of circulating sickle red cells and study the in vivo flow behavior and survival of these subpopulations. 2) Determine the contributions of the K-CI cotransporter and the Gardos channel to the generation of theologically compromised dense sickle cells. 3) Evaluate the contributions of specific adhesive receptors, ligands, and plasma proteins in mediating adhesion of sickle red cells to endothelial cells in vitro. 4) Validate the contribution of various adhesive ligands and receptors to sickle cell adhesion by performing adhesion studies using cells from sickle mice in which genes encoding specific adhesive ligands or receptors are inactivated. For the proposed studies, we will use multiple techniques in biophysics, molecular biology, cell biology, mouse genetics and circulatory physiology. The sickle mice that we developed expressing exclusively human sickle hemoglobin and exhibiting several clinical features of human disease are a key component of the proposed experimental strategy. Our application of novel experimental strategies and exploration of innovative biological concepts offers promise for elucidating the causes of painful vaso-occlusion and chronic organ damage. We anticipate that our findings will lead to the identification of useful therapeutic strategies for the effective clinical management of this debilitating human disease.